An Sn--Pb alloy has been widely used for many years as a soldering alloy for use in carrying out soldering on electronic equipment. An Sn--Pb alloy has a melting point of 183.degree. C. at its eutectic composition (63%Sn-bal.Pb), and it has a soldering temperature of 220-230.degree. C. which is low enough not to thermally damage heat-sensitive electronic devices. In addition, an Sn--Pb alloy exhibits an extremely high level of soldering performance, and it also has the feature that since there is no substantial difference in temperature between the liquidus line and the solidus line, solidification will take place immediately after soldering without resulting in cracking or separation in a soldered area even if vibration or impact is applied to this area during soldering.
Electronic equipment, such as televisions, video machines, radios, tape recorders, computers, and copy machines are discarded when they malfunction, or become old-fashioned. Since each of these apparatuses includes an outer cover and printed circuit boards of a synthetic resin, and electrically-conducting members and an inner frame each made of a metallic material, they cannot be disposed of by burning, and they are mostly disposed of underground in landfills.
Recently, fossil fuels such as gasoline and heavy oil have been consumed in great amounts, and a large amount of sulfur oxides have been released to the atmosphere, resulting in the occurrence of acid rain. Such acid rain water penetrates into the ground and can dissolve the lead which is contained in the soldered portions of electronic equipment which has been disposed of by underground burial, so that the ground water is contaminated by lead. If the contaminated water enters the water supply and is ingested for long periods, a person drinking the water may have a buildup of lead within his body, leading to the possibility of lead poisoning.
Under these circumstances, in the electronic device industry, development of an alloy solder which does not contain lead, i.e., a lead-free alloy solder, is highly desired.
Conventionally, as a lead-free soldering alloy, an Sn--Ag alloy, an Sn--Sb alloy, an Sn--Bi alloy, and an Sn--Zn alloy, all of which contain predominantly Sn, have been proposed.
An Sn--Ag alloy has a melting temperature of 221.degree. C. at its eutectic composition of Sn-3.5%Ag, which exhibits the lowest melting point. The soldering temperature for this alloy composition is 260-270.degree. C. which is relatively high, and when soldering is carried out on some heat sensitive electronic devices, they suffer from thermal damage, resulting in a deterioration in performance and the destruction of the device.
An Sn--Sb alloy has its lowest melting temperature at a composition of Sn-5%Sb. At this composition, the solidus temperature is 235.degree. C. and the liquidus temperature is 240.degree. C., which are rather high. Thus, the soldering temperature for this alloy is 280-300.degree. C., which is higher than that for an Sn-3.5%Ag alloy. In this range, thermal damage to heat sensitive electronic devices is inevitable.
On the other hand, in the case of an Sn--Bi alloy, its eutectic composition is Sn-42%Bi and its eutectic point is 139.degree. C., which is relatively low compared with that of the eutectic Sn--Pb alloy. From this viewpoint an Sn--Bi alloy is applicable as a soldering alloy unless a soldered area is exposed to a high temperature atmosphere over the eutectic point after soldering. However, an Sn--Bi alloy is brittle and hard, and its mechanical properties such as tensile strength and elongation are not satisfactory.
In contrast, an Sn--Zn alloy has a eutectic point of 199.degree. C. at a eutectic composition of Sn-9%zn. Thus, an Sn--Zn alloy is superior to the other alloys with respect to its melting point, i.e., its eutectic point, which is close to the eutectic point, 183.degree. C. for the conventional 63%Sn--Pb soldering alloy. An Sn--Zn alloy is also superior to an Sn--Pb alloy with respect to mechanical strength.
However, the soldering performance of an Sn--Zn alloy is not satisfactory. In order to improve its solderability as well as its mechanical strength, a variety of Sn--Zn system alloys have been proposed, in which optional elements such as Ag, Cu, Bi, In, Ni, and P are added to the base Sn--Zn alloy.
Soldering performance can be achieved using a soldering iron for these Sn--Zn system alloys in the form of wire so long as a suitable composition of a flux is employed.
However, when these Sn--Zn system alloys are used as a solder paste in which the Sn--Zn alloy is pulverized and mixed with a pasty flux to form a solder paste, the soldering performance is not satisfactory. Thus, when soldering is carried out using a solder paste containing Sn--Zn alloys, soldering defects are unavoidable such that an area to be soldered is not completely wetted (dewetting) and voids remain as dotted unsoldered areas under the soldered portion, which appears to the eye to be well soldered.